"It's high season in the nation's national parks as millions of visitors come to see nature. If last year's visitor figures hold up -- and early indications suggest they will -- nine million visitors will see the Great Smoky Mountains, the most visited national park. Three other parks -- Grand Canyon (more than four million visitors in 2011) Yellowstone (about three million) and Acadia (more than two million) -- combined will attract roughly the same number."

"Yet many visitors are likely to find their view of the natural landscapes softened, if not significantly obscured, by haze or smog from civilization.

An amendment to the Clean Air Act passed 35 years ago was designed to ensure that when visitors get to their destinations, they will be able to see the parks' vistas."